Composites based on carbon nanotubes or nanofibers deposited on an activated support for use in catalysis

ABSTRACT

A composite comprising a support activated by impregnation and carbon nanotubes or nanofibers formed by vapor deposition, wherein the weight of said carbon nanotubes or nanofibers formed on the said support is at least equal to 10.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the technical field of composites based oncarbon nanotubes or nanofibers of large specific surface, intended foruse as catalyst supports or as a catalyst for the chemical orpetrochemical industry, in motor vehicle exhaust gas purification, or insatellite propulsion systems. Their advantage is that of combining theintrinsic properties of the base materials of carbon nanotubes ornanofibers with those of easily manipulated macroscopic structures.

PRIOR ART

The catalysts at present used in the fields of the chemical orpetrochemical industry or in motor vehicle exhaust gas purification areessentially in the form of grains, extrusions, cylinders, or monoliths.These materials in the form of grains, extrusions, cylinders ormonoliths may fulfill a catalyst support function, in which case anactive phase is applied to the said support for forming the catalyst.This active phase is often constituted of metals or metal oxides. Thesaid materials may also themselves show catalytic activity, and in thiscase they constitute the catalyst. The search for new catalysts whichare more selective, of better performance, more durable, and morepractical to utilize, concerns both the supports and the active phases.

Nanostructured compounds (mean diameter typically varying between 2 and200 nm) based on carbon, such as nanotubes or nanofibers, have on theone hand great intrinsic mechanical strength, and on the other hand alarge external exchange surface and a good interaction with thedeposited active phase, permitting a strong dispersion of the latter.These new materials thus have physico-chemical properties of interestfor their use in various fields such as catalysis or in reinforcingmaterials.

According to the prior art (see the article by N. M. Rodriguez, A.Chambers and R. T. K. Baker, “Catalytic engineering of carbonnanostructures”, Langmuir Review, vol. 11, pp. 3862-3866, 1995), thesecarbon-based nanostructured compounds are deposited from a gaseous phasecontaining ethylene, or a CO—H₂ mixture, on a substrate constitutedeither by a metallic powder or by a solid silica support impregnatedwith an aqueous solution of iron nitrate which is then calcined andreduced to iron to form an active phase. In both cases, the metal(copper or iron) acts as a catalyst for the formation of nanotubes ornanofibers from a vapor phase. Patent Application WO 01/51201 (HyperionCatalysts International) gives other methods for preparation of carbonnanotubes and nanofibers and indicates their possible use as catalysts.

Methods of manufacturing nanotubes with a single wall are also known, inwhich the nanotubes are deposited by vapor deposition on an aerogel ofalumina having a specific surface (of the order of 600 m²/g) comprisinga growth catalyst of Fe/Mo type (see the article “A scalable CVD methodfor the synthesis of single-walled carbon nanotubes with high catalystproductivity” by Ming Su, Bo Zheng and Jie Liu, Chemical Physics Letters322, pp. 321-326 (2000).

Problem Posed

According to the state of the art, carbon-based nanostructuredcomposites are synthesized only with low yields. Furthermore, theirnanometric size renders their shaping and use difficult, and gives riseto problems of powder generation during transport and loading; itlikewise makes their use impossible in fixed bed reactors, due to chargeloss problems. Consequently, not only is the cost price of thesecompounds high, but also their use as catalyst or catalyst support inindustrial processes is difficult and of little effectiveness.

The present invention has as its object to propose new composites basedon carbon nanotubes or nanofibers which retain the advantages of thesenanotubes or nanofibers, namely their ability to act as a support of acatalytically active phase, and their intrinsic catalytic activity,without having the known disadvantages of the said nanotubes ornanofibers, namely the difficulty of shaping them, the generation ofdust, the difficulty of using them in a fixed bed reactor, and theircost.

OBJECTS OF THE INVENTION

The Applicant has found a new class of composite materials with highspecific surface which may be used as catalysts or as support of theactive phase in various fields such as catalysis, propulsion, andelectrochemistry.

This class of materials consists of a composite comprising an activatedsupport and nanotubes or nanofibers formed by vapor deposition. Thesupport may be a macroscopic support in the form of beads, felts,fibers, foams, extrusions, monoliths, pellets, etc. The support surfaceintended to receive the deposit of carbon nanotubes or nanofibers shouldbe activated beforehand by deposition of an active phase. The saidcomposites combine the advantages acquired on macroscopic supports andthose of isolated nanoscopic compounds which are the carbon nanotubesand nanofibers; in particular, they have a high specific surface.

The first object of the present patent application is a compositecomprising a support activated by impregnation and carbon nanotubes ornanofibers formed by vapor deposition, characterized in that the weightof the said carbon nanotubes or nanofibers formed on the said activatedsupport is at least equal to 10%, preferably greater than 20%, and morepreferably greater than 30% of the total weight of the composite.

Another object of the present invention is the use of a compositecomprising carbon nanotubes or nanofibers, vapor deposited on a supportactivated by impregnation, as a catalyst support for chemical reactionsin a liquid or gaseous environment.

Another object of the present invention is the use of a compositecomprising carbon nanotubes or nanofibers vapor deposited on animpregnation-activated support and an active phase deposited on thesurface of the said nanofibers or nanotubes, as a catalyst for chemicalreactions in a liquid or gaseous environment.

Yet another object of the present invention is the use of a compositecomprising carbon nanotubes or nanofibers vapor deposited on animpregnation-activated support as an electrode in electrochemicalprocesses or devices.

DESCRIPTION OF THE FIGURES

FIG. 1 shows two scanning electron microscope images with the samemagnification (see Example 1). FIG. 1 a sows an activated support madeof carbon felt impregnated with nickel. FIG. 1 b shows the same supportafter growth of the carbon nanofibers.

FIG. 2 shows the pore distribution of two composites according to theinvention (see Example 2).

FIG. 3 shows a scanning electron microscope image of carbon nanofibersformed on the surface of a graphite electrode (see Example 3).

FIG. 4 shows a comparative trial of catalytic decomposition of hydrazinewith a catalyst of the invention and a prior art catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The terms “carbon nanotubes or nanofibers” and “carbon-basednanostructured composites” denote here tubes or fibers of highly orderedatomic structure, hexagonal compounds of the graphite type, which can besynthesized under certain conditions (see the article “Carbon nanotubes”by S. Iijima, MRS Bulletin, pp. 43-49 (1994)). It is known thataccording to the conditions of synthesis by vapor deposition, andparticularly according to the catalysts used, there can be obtainedeither hollow tubes, possibly formed of several concentric tubes ofdifferent diameters, or solid fibers, likewise of filamentary form, butcontaining graphitic carbon in a typically less ordered form. The saidtubes or fibers may have a diameter typically comprised between 2 and200 nm, this diameter being substantially uniform over the whole lengthof each tube or fiber.

Preparation of Composites According to the Invention

The macroscopic support should have sufficient thermal stability in areducing medium, preferably up to at least 1,000° C. It may be in theform of beads, fibers, felts, extrusions, foam, monoliths or beads. Itcan advantageously be chosen from among alumina, silica, siliconcarbide, titanium oxide, zirconium oxide, cordierite and carbon(particularly graphite and active carbon) in the different formsindicated hereinabove. The specific surface of the said macroscopicsupports may very quite widely according to their origins.Advantageously, the said specific surface, determined by the BET methodof nitrogen adsorption at the temperature of liquid nitrogen (StandardNF X 11-621) may represent 1 m²/g to 1,000 m²/g and more especially 5m²/g to 600 m²/g. Supports are preferred having a specific surfacecomprised between 7 m²/g and 400 m²/g.

The support should be activated by deposition of an active growth phase;this favors the growth of carbon nanotubes and nanofibers in thepresence of a mixture containing a hydrocarbon source and hydrogen. Inthe context of the present invention, this deposition is effected byimpregnation of the surface intended to receive the deposit of carbonnanotubes or nanofibers with a solution of one or more salts oftransition metals. In a preferred embodiment of the invention, thismetal is chosen from the group comprising Fe, Ni, Co, Cu; the bi- ortri-metallic mixtures of these metals are likewise suitable. Theconcentration of the active phase, expressed as weight of metal,advantageously represents 0.1 to 20%, more preferably 0.2 to 15%, andyet more preferably 0.5 to 3%, of the weight of the said support. Thelow values are advantageously chosen for supports whose specific surfaceis rather low, while the elevated values are advantageously chosen forthe supports whose specific surface is rather high. According to theApplicant's observations, the use of too great a quantity of metal mayinterfere with the catalytic activity of the composite if the metalsused for the two catalytic functions (catalysis of growth of nanotubesor nanofibers, and catalysis of the chemical reaction aimed at in theindustrial application of the composite) are different.

Due to its activation by deposition of an appropriate active phase, theactivated support catalyses the growth of carbon nanotubes ornanofibers. Such an activated support may be prepared, for example, inthe following manner:

Firstly, the support, present in the form of powder, pellets, granules,extrusions, foam, monoliths, or other agglomerated forms, is impregnatedby means of a solution or a sol formed with a solvent such as water orany other organic solvent such as dichloromethane or toluene and adesired metal or metals in the form of salts.

The thus impregnated support is dried, and the dried support is calcinedat temperatures which may range from 250° C. to 500° C., under or notunder an inert atmosphere. The support is then placed in contact with areducing agent consisting of pure hydrogen, pure or mixed with an inertgas, or any other gaseous source containing a reducing agent. Thereduction is effected at temperatures below 600° C. and preferablycomprised between 300 and 400° C. and for a time comprised between 0.2hours and 3 hours and preferably between 0.5 hour and 1 hour. The stepof reduction may be effected either outside the synthesis reactorfollowed by storage of the resulting solid in air, or directly in thereactor just before the synthesis of the nanotubes or nanofibers.

The growth of the carbon nanotubes or nanofibers is effected bysubjecting the solid to a gas flow containing hydrogen and a carbonsource to a temperature greater than 500° C., preferably comprisedbetween 500 and 1,000° C., and more preferably between 550 and 750° C.,and under a pressure comprised between 1 and 10 atmospheres, andpreferably comprised between 1 and 3 atmospheres. The gas containingfree hydrogen or an inert gas and the gas containing the carbon sourcemay be brought separately into contact with the macroscopic catalyst.However, in order to obtain a very homogeneous gaseous reaction mediumduring contact with the activated support, it is preferable to first ofall mix the gas containing the carbon source with the gas containinghydrogen or the inert gas and to bring the thus constituted mixture intocontact with the activated support.

The gas containing free hydrogen or an inert gas is used in a suitablequantity to provide a molar ratio H2:C going from 0.5 to 10, andpreferably from 0.1 to 1 in the reaction medium coming into contact withthe activated support. The carbon source may be any molecule containingat least one carbon atom, but is preferably a hydrocarbon, or carbonmonoxide diluted in a stream of inert gas in the presence of hydrogen.The hydrocarbon may be any saturated or olefin, C1-C6 hydrocarbon,preferably a C1-C4 saturated hydrocarbon, and more preferably asaturated hydrocarbon of chain length comprised between C1 and C3.Methane and ethane are preferred among these different families ofhydrocarbons.

The contact time between the reagents and the solid I comprised between0.5 second and several minutes. preferably between 0.5 and 60 secondsand more preferably between 1 and 30 seconds. The total pressure of thesynthesis may be variable and comprised between 1 and 10 atmospheres,preferably between 1 and 5 atmospheres and more preferably between 1 and3 atmospheres. The duration of the synthesis is comprised between 1 hourand 24 hours, preferably between 2 hours and 12 hours and morepreferably between 2 hours and 6 hours. In a preferred embodiment, thisduration is chosen such that the quantity deposited in the form ofcarbon nanotubes or nanofibers, in weight of carbon, is at least fivetimes, and preferably between twenty times and a hundred times, and yetmore preferably between fifty times and a thousand times, greater thanthe weight of the active phase, expressed in weight of metal.

After synthesis, the solid is cooled under the reaction medium to 200°C.; the mixture is then replaced by pure hydrogen down to ambienttemperature. The solid is then discharged and stored under air atambient temperature.

Characteristics and Advantages of Composites According to the Invention

The morphology of the carbon nanotubes or nanofibers according to theinvention is characterized by nano-structured carbon in the form ofnanotubes or nanofibers of mean diameter comprised between 5 nm and 200nm. The mean diameter of the carbon nanotubes or nanofibers may varyquite widely according to the starting catalysts used in the activephase, and according to the synthesis conditions. Advantageously, thesaid diameter, determined by scanning and transmission electronmicroscopy, varies between 0.01 micrometer and 20 micrometers, and morepreferably 0.05 micrometers and 10 micrometers. The mean length of theseand tubes is located between several tens and several hundreds ofmicrometers. The macroscopic morphology of the starting supports ispreserved.

The specific surface, measured by the BET method of nitrogen adsorptionat the temperature of liquid nitrogen (NF X 11-621 Standard), ofcomposite materials according to the invention, is typically comprisedbetween 1 and 1,000 m²/g; it is preferably greater than 10 m²/g. Formost of the envisaged industrial applications, composites may be usedwith a value comprised between 10 m²/g and 100 m²/g. Their poredistribution is essentially mesoporous, with a mean size comprisedbetween 5 and 60 nm. It is preferable that the microporous surface isthe smallest possible, and represents less than 10% of the totalcontribution of the surfaces.

In a preferred embodiment of the invention, the weight of the carbonnanotubes or nanofibers formed on the support is at least equal to 10%,preferably greater then 20% and more preferably greater than 30% of thetotal weight of the composite.

The hardness of the composites according to the invention is distinctlyhigher than those of the support materials, because the formation ofcarbon nanotubes or nanofibers at the surface and in the matrices of thesaid starting materials.

Advantages of the Composites According to the Invention

The composites according to the invention have numerous advantages, withrespect on the one hand to the known supports, and on the other handwith respect to the known carbon nanotubes or nanofibers. Theirmanipulation is easy because the macroscopic form of the support ispreserved, the deposited carbon nanotubes or nanofibers not in any waymodifying the morphology of the support. Their external exchange surfaceis large, in the same way as their specific surface, with respect tothat of the starting solid, because of the presence of carbon nanotubesor nanofibers on the external surface. Their thermal and electricalconductivities are good, due to the presence of carbon nanotubes ornanofibers at the surface (case of monoliths). The strong interactionbetween the carbon nanotubes or nanofibers and the precursor salts ofthe active phase ensures a good dispersion of the latter. Due to thestrong interaction between the nanotubes or nanofibers and themacroscopic support, the problem does not arise of the generation ofdust during the manipulation of these materials, which is one of thedisadvantages of the known carbon-based nanostructures; this absence ofpowder likewise facilitates the separation of the catalysts and thereaction products, which is a property of prime importance for liquidphase reactions. Likewise, the small size of the carbon nanotubes ornanofibers permits considerable reduction of weight transfer phenomena.The composites according to the invention furthermore have very strongresistance as regards problems of sintering caused by steam orthermally, compared with that of the conventional solid oxide supportssuch as alumina (Al2O3), silica (SiO₂), TiO₂, or ZrO₂.

All these mew properties confer on the composites according to theinvention a strong [potential for application in various fields, such ascatalysis, propulsion, and electrochemistry, or in the fields ofmechanical reinforcement if materials working under high stress or underfriction.

Industrial Applications of the Composites According to the Invention

The composites according to the invention may have numerous industrialapplications. They can be used as catalyst supports or directly ascatalysts of chemical reactions in chemical industry, petrochemicalindustry, or in the purification of motor vehicle exhaust gas. They haveincreased resistance, both mechanical and chemical, under workingconditions in the presence of a high steam pressure or in a moistatmosphere.

By way of example, the composites according to the invention maydirectly catalyze Friedel-Crafts acylation in a liquid medium.

After application of an appropriate active phase, they may catalyze thedecomposition of hydrazine and its derivatives and of hydrogen peroxide(active phase: Ir (preferably) or Ru), ammonia synthesis in the presenceof N2 and H2 (active phase: Ru (preferably) or Fe), selective or totaloxidation such as CO oxidation to CO₂ (active phase: Ni or Fe),hydrogenation-dehydrogenation, such as the hydrogenation ofnitro-aromatics or of aromatics (active phase: Pt or Pd).

The Applicant has shown that the composites according to the inventionmay be used as an electrode in electrochemical processes or devices.

Due to their increased mechanical strength, they may be used in fieldsother than catalysis, for example, as reinforcing materials in materialsworking under strong frictional stress. Furthermore, the deposition ofthe carbon nanotubes or nanofibers considerably increases mechanicalstrength against crushing of the final composite with respect to that ofthe starting material: this surface deposit according to the inventionmay thus act as a surface treatment to protect the substrate. Thus,composites according to the invention may be used as reinforcement orprotection of materials or elements working under friction.

EXAMPLES

To complete the preceding description, a series of non-limiting examplesillustrating the invention are given below.

Example 1

Preparation of a Composite Based on Carbon Nanofibers Deposited onCarbon Felt

The support consisting of carbon felt is composed pf a network of carbonfibers having an external diameter centered on 0.01 mm and a specificsurface measured by the BET method, of 10 m²/g. The felts are firsttreated in a mixture of aqua regia (HCl, HNO3) at ambient temperaturefor 6 hours, to prepare their surface (which is originally hydrophobic)for impregnation.

Nickel in the form of nitrate (with distilled water as solvent) oracetylacetonate (with toluene as solvent) is deposited on the surface bysuccessive impregnations. The impregnated supports are then dried in airat 100° C. for 6 hours, followed by calcinations in air at 400° C. for 2hours with transformation of the nickel salts into oxide.

The samples are then placed in a tubular oven and swept with a stream ofargon at ambient temperature for 1 hour. The argon is replaced withhydrogen, and the temperature is progressively raised from ambient to400° C. (heating gradient 5° C./min) and kept at this temperature for 2hours; the nickel oxide is reduced to metal. The temperature is thenraised from 400° C. to 700° C. and the stream of hydrogen is replaced bythe reaction mixture containing hydrogen and ethane. The total flow rateis fixed at 150 ml/min (H2: 100 ml/min and C2H6: 50 ml/min). Theduration of the synthesis is fixed at 6 hours. After synthesis, thesamples are cooled under the reaction mixture to 200° C., and thereaction mixture is replaced by pure hydrogen.

FIG. 1 a shows a scanning electron microscope image of carbon felt,impregnated beforehand with nickel. The diameter of the fibers formingthe felt is centered around 10 micrometers. FIG. 1 b shows themorphology of the composite, carbon nanofibers on carbon felt, obtainedafter growth under a stream of hydrogen and ethane at 700° C. Thediameter of the filaments in the composite is greatly increased; it isnow of the order of four times greater than that of the startingfilaments (FIG. 1 a). The presence of carbon nanofibers is clearlyvisible in the form of small filaments. Observation at greatermagnification gives a diameter of the nanofibers between 80 and 100 nm.

The different characteristics of the samples obtained at 700° C. andwith a synthesis duration of two and six hours are shown in Table 1. Thegain in weight resulting from the formation of carbon nanofibers on thesurface of the graphite felts varies little as a function of the nickelconcentration, while the duration of the synthesis has an influence, notnegligible, on the specific surface of the carbon nanofibers. Theformation of carbon nanofibers on the surface of the supportsignificantly increases the specific surface, measured by the BETmethod, of the samples as well as their porosity, essentially in theregion of the mesopores between 3 and 20 nm, as is shown by the poredistribution of composites based on carbon nanofibers deposited ongraphite felts (FIG. 2) TABLE 1 Characteristics of composites obtainedin Example 1 (Conditions: 700° C., H₂: 100 ml/min, C₂H₆: 50 m;/min)Metal Gain in Specific Pore content (% weight (% surface volume Sampleby weight) Duration (h) by weight) (m²/g) (ml/g) 1 0.5 2 58 43 0.12 20.5 6 86 34 0.13 3 1 2 52 50 0.14 4 1 6 94 42 0.14

This increase of surface is attributed to the formation of carbonnanofibers on the surface of the starting macroscopic support. The basalplanes of the graphite present in the nanofibers contribute to theaugmentation of the specific surface observed in the composites.Nevertheless, the total specific surface of the composites decreasessubstantially when the duration of the synthesis goes from 2 hours to 6hours.

The mechanical hardness of the composites obtained is greatlyameliorated with respect to those of the starting graphite felt.

The strength of the carbon nanofibers formed on graphite felts ischaracterized by subjecting the composite obtained to a sonication in anultrasonic water bath for a duration of at least half an hour with anominal power of 1,100 W at a frequency of 35 kHz. The absence ofresidues in the solution indicates that the fibers are not detached fromthe surface of the composite during the operation, thus indicating thegreat resistance to attrition of the composite.

Example 2

Preparation of a carbon nanofibers deposited on a monolithic TiO₂support

The TiO₂ monolith is characterized by square channels of 3 mm side and awall thickness of about 0.5 mm. The starting material has a specificsurface, measured by nitrogen absorption, of the order of 100 m3/g.Before synthesis, the material is subjected to a thermal treatment at700° C.; the phenomena of sintering and of phase transition lead to anon-negligible loss of the initial specific surface, which becomes 45m²/g (Table 2). The synthesis is performed under a mixture containing100 ml/min of hydrogen and 100 ml/min of ethane. The duration of thesynthesis is fixed at two hours. After synthesis, the samples are cooledunder reaction mixture to 200° C., then under hydrogen to ambienttemperature.

The physical characteristics of the samples obtained at 700° C. and twohours of synthesis are shown in Table 2. The formation of carbonnanofibers on the surface of the monolithic support considerablyincreases its specific surface, from 45 m²/g to 100 m²/g. The poredistribution was likewise modified due to the formation of carbonnanofibers. TABLE 2 Characteristics of the composites obtained inExample 2 Metal Specific Pore content (% T surface volume Sample byweight) Duration (h) (° C.) (m²/g) (ml/g) 1 0 2 700 46 0.22 2 1 2 700 980.17 3 1 2 700 45 0.12 4 1 6 800 50 0.09 5 2 2 700 88 0.11 6 2 2 800 550.12 7 2 12 700 59 0.09

The pore distribution of the composites obtained is shown in FIG. 3 as afunction of the temperature and duration of the synthesis. In this caseit is likewise to be noted that the formation of carbon nanofibers hassignificantly contributed to the mechanical behavior of the saidcomposites.

Example 3

Preparation of a composite of carbon nanofibers deposited on a vitreouscarbon disk for electrochemical applications

A disk of vitreous carbon, 2 cm in diameter and 0.4 cm thick, is firstwashed by dipping into an aqua regia mixture HCl/HNO₃), followed bycopious rinsing with distilled water and drying at 100° C. One surfaceof the disk is then impregnated by deposition of aqueous nickel nitratesolution (1.4 mg/1 ml), followed by evaporation of the water at 100° C.under air overnight.

The sample is then calcined at 300° C. for two hours in air, then for anhour at 400° C. under a stream of hydrogen. The formation of nanofibersis obtained by treating the sample under a stream containing a mixtureof hydrogen (100 ml/min) and ethane (50 ml/min) at 650° C. for 2 hours.During this step, the sample is placed horizontally in the tubular ovenwith the nickel-treated surface upward. After synthesis, the sample iscooled under the reaction mixture to 200° C., then under a stream ofpure hydrogen to ambient temperature. The sample is then discharged,then stored in air.

The composite thus obtained is composed of one smooth surface, while theother surface has excrescences visible to the naked eye. The weight ofthe disk has increased by 10%. FIG. 3 shows a scanning electronicmicroscope image entanglements of carbon nanofibers whose diametervaries of carbon nanofibers formed on the surface of a graphiteelectrode under a stream of hydrogen and ethane at 650° C. The formationis to be distinguished on the disk surface of from several tens ofnanometers to several hundreds of nanometers. These modified electrodesare electroactive. In particular, catalytic currents corresponding tothe reduction of CO₂ to CO have been measured by cyclic voltammetryunder Ar and under CO₂ in acetonitrile containing 20% of water and in apurely aqueous medium. The current densities i are between 1.6 mA/cm2and 10.6 mA/cm2.

Example 6

Preparation of a composite of carbon nanotubes deposited on a carbonfelt support

The support is similar to that of Example 1, and is prepared by the sameprocedure. The nanotube growth catalyst is iron, which is deposited ofthe carbon felt following the same mode of impregnation as that used inExample 1. The thermal treatments are likewise identical.

The samples are then placed in a tubular oven and swept with a stream ofargon at ambient temperature for 1 hour. The argon is replaced byhydrogen, and the temperature is raised progressively from ambient to400° C. (heating gradient 5° C./min) and kept at this temperature for 2hours; the iron oxide is reduced to metal. The temperature is thenraised from 400° C. to 750° C. and the stream of hydrogen is replaced bythe reaction medium containing hydrogen and ethane. The total throughputis fixed at 100 ml/min (H2: 50 ml/min and n-C₂H₆: 50 ml/min). Theduration of the synthesis is fixed at 6 hours. After synthesis, thesamples are cooled under the reaction mixture to 300° C., and themixture is then replaced with pure hydrogen.

The composite obtained has the same characteristics as that based oncarbon nanofibers described in example 1. The total specific surface ofthe composite is slightly lower and varies between 20 and 40 m²/g.

Example 5

Acylation of anisole by benzoyl chloride on composite based on carbonnanotubes

The composite of carbon nanotubes on graphite felt is used, without anytreatment beforehand, in the reaction of acylation of anisole by benzoylchloride according to the following equation:

The reagents are dissolved in a solution of chlorobenzene. Theconcentration of anisole is 2 millimoles and that of the benzoylchloride is 1 millimole. The solution is then degassed under a stream ofargon at ambient temperature for 30 minutes. 0.2 grams of the compositeare introduced into the flask, then the atmosphere of the reactor ispurged under a stream of argon at ambient temperature for 30 minutes.The flask is closed and the temperature is brought to 120° C. Theacylation is followed by gas phase chromatography. The results obtainedas a function of time are reported in Table 3. TABLE 3 Friedel-Craftsreaction on composite based on carbon nanotubes Time under streamAnisole Ketone (hours) (mol. %) (I) (mol. %) Ester (II) (mol. %) 0 100 189.6 4.0 6.4 2 69.9 19.7 10.4 3 56.3 31.4 12.3 4 53.7 34.5 11.8 5 42.446.1 11.5 8 28.4 60.2 11.4

Example 6

Decomposition of hydrazine

The decomposition of hydrazine is a process used industrially insatellite propulsion systems. The principal industrial catalyst isIr-37%/alumina.

A composite of carbon nanofibers on carbon felt was prepared accordingto a procedure similar to those of the preceding examples. The compositewas subjected to a sonification (1,100 W, 35 kHz) for 30 minutes inorder to eliminate any fibers which are not well attached to the feltsurface. 267 mg of this composite were impregnated with a solution ofH₂IrCl₆.6H₂O containing 215 mg of iridium. The product was dried at 100°C. and then heated to 300° C. for 2 hours in order to transform theiridium salt into oxide. The oxide was then reduced under a stream ofhydrogen at 400° C. The final catalyst thus obtained (catalyst A)contained 30% metallic iridium by weight.

The trials of hydrazine decomposition were performed by injection of 0.4ml of 99.9% pure hydrazine into a reactor which contained the samequantity (120 mg) of catalyst, namely either the catalyst A (accordingto the invention, containing 30% by weight of metallic iridium), or aniridium-based catalyst (37% by weight of metallic iridium) on aluminaaccording to the prior art (catalyst B). The results are shown in FIG.4. It can be seen that, for the same quantity of catalyst (correspondingto a very similar metallic iridium content)and the same quantity ofinjected hydrazine, the pressure of the gas generated by thedecomposition of the hydrazine is about 3 times greater with catalyst A(according to the invention) than with catalyst B (according to theprior art). This decomposition is provided in an interval of timesufficiently short to permit the use of the catalyst according to theinvention in a propulsion system, for example for the precisepositioning of satellites.

1. A composite comprising: a support activated by impregnation andcarbon nanotubes or nanofibers formed by vapor deposition, wherein theweight of said carbon nanotubes or nanofibers formed on the said supportis at least equal to 10%.
 2. Composite according to claim 1, whereinsaid support is in the form of beads, fibers, felts, extrusions, foams,monoliths or pellets, and said support is activated by impregnation ofan aqueous solution of one or more transition metals, followed bydrying, calcination and a reducing treatment by placing said support incontact with a reducing gas.
 3. Composite according to claim 1, saidsupport is selected from the group consisting of alumina, silica,titanium oxide, zirconium oxide, cordierite, and carbon.
 4. Compositeaccording to claim 2, wherein said support has, before impregnation, aBET surface comprised between 1 m²/g and 1,000 m²/g, and preferablycomprised between 5 m2/g and 600 m²/g.
 5. Composite according to claim4, wherein said support has, before impregnation, a BET surfacecomprised between 7 m²/g and 400 m²/g.
 6. A composite comprising carbonnanotubes or nanofibers vapor deposited on a support activated byimpregnation, wherein said composite is capable of functioning as asupport for a catalyst of chemical reactions in liquid or gaseous media.7. A composite comprising carbon nanotubes or nanofibers vapor depositedon a support activated by impregnation and an active phase deposited onthe surface of the said nanotubes or nanofibers, wherein said compositeis capable of functioning as catalyst of chemical reactions in a liquidor gaseous medium.
 8. A composite comprising carbon nanotubes ornanofibers formed by vapor deposition on a support activated byimpregnation wherein said composite is capable of functioning as anelectrode in electrochemical processes or devices.
 9. A compositecomprising carbon nanotubes or nanofibers formed by vapor deposition ona support activated by impregnation wherein said composite is capable offunctioning as reinforcement or protection of materials or elementsworking under friction.
 10. A composite comprising according to claim 6,wherein said support intended to be activated by impregnation isselected from the group consisting of alumina, silica, silicon carbide,titanium oxide, zirconium oxide, cordierite, and carbon.
 11. A compositeaccording to claim 6, wherein said support is in the form of beads,fibers, felts, extrusions, foam, monoliths, or pellets.
 12. A compositeaccording to claim 6, wherein said support has, before impregnation, aBET surface comprised between 1 m²/g and 1,000 m²/g.
 13. A compositeaccording to claim 6, wherein said surface of said support intended toreceive the deposit of carbon nanotubes or nanofibers has beenimpregnated with a solution of one or more transition metal salts.
 14. Acomposite according to claim 13, wherein said transition metal isselected from the group consisting of Ni, Fe, Co and Cu.
 15. A compositeaccording to claim 13, wherein the impregnation is followed by drying,calcinations and a reducing treatment by placing said support in contactwith a reducing gas.
 16. A composite according to claim 6, wherein saidcarbon nanotubes or nanofibers have been formed starting from a gaseousmixture containing molecular hydrogen or an inert gas, and a gascontaining carbon.
 17. A composite according to claim 16, wherein saidgas containing carbon contains carbon monoxide or a hydrocarbon.
 18. Acomposite according to claim 6, wherein the weight of said carbonnanotubes or nanofibers formed on said support is at least equal to 10%of the total weight of the said composite.
 19. A composite according toclaim 6, wherein the specific surface of said composite, determined bythe BET method of adsorption of nitrogen at the temperature of liquidnitrogen according to the Standard NF X 11-627, is greater than 10 m²/g.20. A composite according to claim 6, that is capable of functioning asa catalyst or catalyzer support in the chemical industry, petrochemicalindustry, and/or in purification of motor vehicle exhaust gas.
 21. Acomposite according to claim 6, that is capable of functioning as acatalyst or catalyst support, wherein said chemical reactions in liquidor gaseous medium are selected from the group consisting ofFriedel-Crafts acylation, decomposition of hydrazine and itsderivatives, decomposition of hydrogen peroxide, ammonia synthesis inthe presence of N2 and H2, oxidation of CO to CO₂.
 22. A compositeaccording to claim 21 for the decomposition of hydrazine, wherein anactive phase of iridium has been deposited beforehand on the surface ofthe said carbon nanotubes or nanofibers.
 23. A composite according toclaim 22 in a propulsion system for satellites.
 24. A process forpreparing a composite comprising a support activated by impregnation andcarbon nanotubes or nanofibers created by steam deposition, said processcomprising: (i) depositing an active phase on said support byimpregnation, (ii) drying and calcining the impregnated support, andperforming a reduction treatment by placing said support in contact witha reducing gas, (iii) forming said carbon nanotubes or nanofibers bysteam deposition using a gaseous mixture containing molecular hydrogenor an inert gas, and ethane.
 25. The process according to claim 24,wherein said support is selected from the group consisting of alumina,silica, titanium oxide, zirconium oxide, cordierite and carbon.
 26. Theprocess according to claim 24, wherein prior to impregnation, saidsupport has a BET surface area ranging from 1 m²/g to 1000 m²/g.
 27. Theprocess according to claim 24, wherein said support is in the form ofbeads, fibers, felt, extrusions, foam, monoliths and/or chips.
 28. Amethod for preparing a catalyst or catalyst support comprising using acomposite containing carbon nanotubes or nanofibers steam deposited on asupport activated by impregnation, said catalyst or catalyst supportbeing suitable for use with chemical reactions in a liquid or gaseousmedium, wherein said chemical reactions are selected from among thegroup consisting of: depolluting exhaust gas from motor vehicles,Friedel-Crafts acylation, the decomposition of hydrazine and itsderivatives, the decomposition of hydrogen peroxide, the synthesis ofammonia in the presence of N₂ and H₂, the oxidizing of CO in CO₂, andthe hydrogenation of nitroaromatics or aromatics.
 29. A method accordingto claim 28 for hydrazine decomposition, wherein an active iridium phasehas been previously deposited on the surface of said carbon nanofibersor nanotubes.
 30. A method according to claim 29 in a satellitepropulsion system.
 31. A method according to claim 28, wherein saidsupport intended to be activated by impregnation is selected from thegroup consisting of alumina, silica, silicon carbide, titanium oxide,zirconium oxide, cordierite and carbon.
 32. A method according to claim28, wherein said support is in the form of beads, fibers, felt,extrusions, foam, monoliths and/or chips.
 33. A method according toclaim 28, wherein said support, prior to impregnation, has a BET surfacearea ranging from 1 m²/g to 1000 m²/g.
 34. A method according to claim28, wherein the surface of said support intended to receive thedeposited carbon nanofibers or nanotubes has been impregnated with asolution of one or more transition metal salts selected from the groupconsisting of Ni, Fe, Co and Cu.
 35. A method according to claim 28,wherein impregnation is following by drying, calcination and a reducingtreatment by contacting a reducing gas.
 36. A method according to claim28, wherein said carbon nanofibers or nanotubes have been formed of agaseous mixtures containing molecular hydrogen or an inert gas, and agas containing ethane.
 37. A method according to claim 28, wherein theweight of said carbon nanofibers or nanotubes formed on said support isat least 10% of the total weight of said composite.
 38. A methodaccording to claim 28, wherein the specific surface of said composite,as determined by the BET nitrogen adsorption method at the temperatureof liquid nitrogen according to standard NF X 11-621 is greater than 10m²/g.